1. Field of the Invention
This invention relates to a method for improving and stabilizing the brightness and strength of felted cellulosic sheet material by treating the material with a solution of an alkali metal borohydride, and thereby retarding the type of deterioration which results in color formation in and embrittlement of the cellulosic material. The invention also contemplates improving the results obtained from the borohydride treatment by washing the material with a solution containing an alkaline earth metal hydroxide, carbonate, or bicarbonate. The invention further relates to an apparatus for efficiently washing the material in a continuously flowing stream of wash medium.
2. Prior Art
It is well known to investigators in the art that acidic paper or other felted cellulosic sheet material, having an initial pH of below 5.0, generally degrades rapidly. Such acidic cellulosic material becomes discolored and brittle and may have a useful life of only 30 to 80 years. In contrast, a similar paper of felted cellulosic material having an initial pH above 6.5 generally exhibits a slow rate of deterioration. Such neutral or slightly alkaline felted cellulosic material remains bright and flexible on natural aging. Machine-made paper produced since the early years of the 19th century is frequently acidic, with an initial pH value in the range of 4.5 to 5.5, because of the use of alum/rosin sizing used in its manufacture. Gaseous oxides of sulfur and nitrogen present as pollutants in urban atmospheres are absorbed by cellulosic materials and react with ambient moisture to form acids which cause further deterioration of these materials.
The stability of a material is reflected by the extent to which the original properties of the material are retained over time. A stable felted cellulosic sheet material will exhibit a low rate of change in properties such as brightness and strength as measured by folding endurance. It is established conservation practice to improve the brightness and strength stability of felted cellulosic sheet material by neutralizing acidity produced from internal sources created by the manufacturing procedures and deacidifying felted cellulosic sheet material by neutralizing existing acidity and incorporating into the material an alkaline reserve to neutralize acidity absorbed into cellulosic material over the course of time from external sources such as air pollutants.
In addition to the deterioration associated with the presence of acidity in paper, oxidative reactions are an important component of cellulose degradation, and may be manifested by color formation and embrittlement. Pulping and bleaching procedures used during the manufacture of felted cellulosic material may introduce oxidized groups along the cellulose molecule which groups, in turn, may be involved in color formation, chain scission, and depolymerization reactions. In particular carbonyl groups, especially those occurring at carbon atoms 2 and 3 of oxidized anhydroglucose units within the cellulose molecule, have been implicated in color reversion. Trace metals from machinery or from natural water sources may further act as oxidation catalysts in the resulting felted cellulosic material.
In aqueous solution, alkali metal borohydrides, particularly sodium and potassium borohydride, are moderate reducing agents which can selectively reduce carbonly compounds such as aldehydes and ketones to the corresponding alcohols, without reducing carboxylic acid groups. The alkali metal borohydrides also act as reducing agents with a variety of inorganic metal ions in aqueous solution.
William H. Rapson, in Canadian Pat. Nos. 610,654 and 610,655, both issued Dec. 13, 1960, proposes using sodium borohydride in processing cellulosic pulps, in order to increase the brightness and improve the brightness retention of such pulps. In Canadian Pat. No. 610,654, Rapson uses sodium borohydride as a supplemental treatment following chlorine dioxide bleaching of wood pulp. Rapson notes that while chlorine dioxide readily oxidizes lignin and resin, it does not react with the carbohydrate component of wood pulp. Thus, while chlorine dioxide does not introduce undesirable carbonyl groups into the cellulose fraction, neither does it oxidize nor thereby eliminate those carbonyl groups already present. The combination of chlorine dioxide followed by sodium borohydride is used by Rapson in order to produce a pulp of high whiteness and color stability, without the loss in pulp strength which would occur if a strong oxidizing bleach such as hypochlorite were used. In Canadian Pat. No. 610,655, Rapson used sodium borohydride treatment to increase the brightness and improve the color stability of wood pulp and of cotton linters. Rapson also reports that sodium borohydride treatment improves the color stability of cotton linters which have been bleached with sodium hypochlorite at pH 7. In both Canadian Pat. Nos. 610,654 and 610,655, Rapson only suggests that sodium borohydride treatments are useful during the processing of cellulosic pulps. Rapson does not suggest the present invention wherein alkali metal borohydride is adapted to successfully treat formed felted cellulosic sheet material, including aged papers.
Francis K. Burr, in U.S. Pat. No. 3,081,265, issued Mar. 12, 1963, proposes using an alkali metal borohydride as part of a detergent composition for washing cotton or rayon fabrics, in order to decrease the loss in strength of such woven fabrics after repeated washings. Burr suggests that the detergent composition is particurlarly effective for repeated washing of cotton or rayon fabrics in water containing iron, because the composition prevents the deposition onto the fabric of iron which otherwise would cause discoloration and contribute to lower strength. Burr only suggests that the incorporation of sodium borohydride in an aqueous solution of detergent is useful during repeated washings of woven cotton or rayon textile fabrics. Burr does not suggest the present invention wherein alkali metal borohydride is adapted to successfully brighten and strengthen nonwoven felted cellulosic sheet material such as paper.
Lucia C. Tang and Norvell M. M. Jones in the JOURNAL OF THE AMERICAN INSTITUTE FOR CONSERVATION, 18 (2), pp. 61-81 (1979), note that calcium ion in the wash water used in the conservation of paper materials improves the aging characteristics of such paper. Tang and Jones suggest introducing calcium ion into laboratory distilled or demineralized wash water by flowing the deionized water through a column of calcium carbonate. Tang and Jones do not attempt to control the quantity of calcium ion so introduced into the wash water.